1,2,2,2-tetrafluoroethyl ethers, and liquid-crystalline medium

A 1,2,2,2-Tetrafluoroethyl ether of formula I

• R--(A.sup.1 --Z.sup.1).sub.m --A.sup.2 --Z.sup.2 --A.sup.3 --O--CHF--CF.sub.3 I
• in which
• R is H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF.sub.3 or at least monosubstituted by halogen, and one or more CH.sub.2 groups in these radicals is optionally replaced, in each case independently of one another, by --O--, --S--, ##STR197## --CO--, --CO--O--, --O--CO-- or --O--CO--O-- in such a way that O atoms are not linked directly to one another,
• A.sup.1, A.sup.2
• and A.sup.3 are each, independently of one another,
• (a) a trans-1,4-cyclohexylene radical in which, in addition, one or more non-adjacent CH.sub.2 groups is optionally replaced by --O-- and/or --S--,
• (b) a 1,4-phenylene radical in which, in addition, one or two CH groups is optionally replaced by N,
• (c) a radical from the group consisting of 1,4-cyclohexenylene, 1,4-bicyclo[2.2.2.]octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
• where the radicals (a) and (b) are optionally substituted by one or two fluorine atoms,
• Z.sup.1 and Z.sup.2 are each, independently of one another, --CO--O--, --O--CO--, --CH.sub.2 O--, --OCH.sub.2 --CH.sub.2 CH.sub.2 --, --CH.dbd.CH--, --C.tbd.C-- or a single bond, one of the radicals Z.sup.1 and Z.sup.2 also being --(CH.sub.2).sub.4 -- or --CH.dbd.CH--CH.sub.2 CH.sub.2 --, and
• m is 0,1 or 2.